1. Field of the Invention
The present invention relates to a method of producing a ceramic and, more particularly, to a method of producing an oriented ceramic which can be especially used as an electronic material such as a piezoelectric material or the like.
2. Description of the Related Art
According to one method of producing ceramics in the field of the present invention, ceramic green sheets are laminated, press bonded to each other and fired. The ceramic green sheets are pressed in this method in such a manner that the area of the respective ceramic green sheets in the direction perpendicular to the pressing axis is prevented from increasing. The crystal grains of the ceramic obtained by this method are not oriented.
On the other hand, it is known that oriented ceramics in which the crystal grains are oriented are especially useful as electronic materials such as piezoelectric materials or the like. For example, as described in the report by T. Takenaka, et al., the orientation of a layered perovskite compound ceramic such as Na0.5Bi4.5Ti4O15 or the like as a piezoelectric material caused the electromechanical coupling coefficient for the thickness longitudinal fundamental vibration of a columnar vibrator to increase to about 2.2 times of that of the ordinary not-oriented ceramic (Sensor and Materials. Vol. 1, 35 (1988)). S. Jin et al reported that as a superconductor material, an oriented YBa2Cu3O7-xcex4 ceramic was prepared, and the critical current density thereof was increased to about 12 times of that of the non-oriented ceramic (Physical Review B, vol. 37, No. 13, 7850 (1988)). It will be appreciated that orientation means the state of crystal grains having a large shape-anisotropy in which the directions of the grains are the same as a whole.
As methods of producing oriented ceramics, hot forging, Templated Grain Growth (TGG), and so forth have been employed.
Oriented ceramics having high orientation degrees can be obtained by the hot forging method. T. Takenaka et al produced an oriented ceramic of Na0.5Bi4.5Ti4O15 by the hot forging method. According to this method, a formed product is heat-treated (fired) while it is pressed. The orientation degree of the produced oriented ceramic, measured by the Lotgering method, was 98%. However, the hot forging method needs to employ a special heat-treatment apparatus suitable for press-firing and, moreover, is a batch-process heat-treatment. Thus, this method is expensive and unsuitable for mass production.
Seong-Hyon Hong et al produced an oriented ceramic of Bi4(Ti3.06Nb0.04)O12 by the TGG method. Here, the ceramic crystal grains having a shape-anisotropy are mixed prior to forming. The orientation degree of the oriented ceramic obtained by this method, measured by the Lotgering method, was 96%, and the piezoelectric constant d33 thereof was enhanced to about 1.5 times of that of the non-oriented ceramic (J. Am. Ceram. Soc., vol. 83, 113 (2000)). It is unnecessary to press-fire by a batch-process according to the TGG method, and therefore, this method is suitable for mass production. However, the orientation degree of the crystal grains of a ceramic produced by the TGG method is low compared to that by the hot forging method.
To enhance the characteristics of a ceramic such as an electromechanical coupling coefficient by orienting the ceramic, it is necessary to realize a still higher orientation degree. In general, it is more difficult to produce highly-oriented ceramics by the TGG method compared to the production by the hot forging method.
The inventors compared a non-oriented ceramic prepared by press-bonding a laminate and then firing with the hot forging and TGG methods using CaBi4Ti4O15+0.5% by weight MnCO3. Table 1 shows the comparison results.
As seen in Table 1, the crystal grains of the ceramics produced by the hot forging and TGG methods are oriented in contrast to the ceramic produced by the prior art method of firing a press-bonded laminate. The orientation degree of the ceramic or produced by the TGG method is lower, and also, the enhancement of the characteristic is smaller, compared to those by the hot forging method.
Accordingly, it is a main object of the present invention to provide a method of producing a ceramic by which an ordinary baking furnace can be used for firing, and in the case of the same materials being used, an oriented ceramic having an orientation degree higher than that made by the TGG method can be produced.
It is another object of the present invention to provide a method of producing a ceramic by which an ordinary baking furnace can be used for firing, and in the case of the same materials being used, an oriented ceramic having an orientation degree higher than that by the TGG method and substantially equal to that by the hot forging method can be produced.
It is still another object of the present invention to provide a method of producing a ceramic by which an ordinary baking furnace can be used for firing, and in the case of the same materials being used, an oriented ceramic which has a higher orientation degree and a higher sintering density than those by the TGG method can be produced.
Specifically, the present invention provides a method of producing a ceramic comprising the steps: preparing ceramic slurry containing a powder of ceramic crystal grains having a shape-isotropy mixed with a powder of a ceramic raw material or a calcined powder of a ceramic raw material, or both; forming the ceramic slurry to produce a formed product; uniaxially pressing the formed product so that the length of the formed product in the direction parallel to the pressing axis is decreased compared to that before the pressing, and the area of a plane perpendicular to the pressing axis of the formed product is increased compared to that before the pressing, whereby an oriented formed product is produced; and firing the oriented formed product to sinter it.
Preferably, the length of the oriented formed product in the direction parallel to the pressing axis is up to about half of the length of the formed product before pressing.
Also, preferably, the amount of the ceramic crystal grains having a shape-anisotropy is in the range of about 25 to 52% by weight based on 100% by weight of the mixed powder.
Furthermore, preferably, the ceramic crystal grains having a shape-anisotropy are flat, and the aspect ratio is in the range of about 5 to 10. The aspect ratio is the ratio of the maximum size of a ceramic crystal grain to the height thereof.
Preferably, the ceramic crystal grains having a shape-anisotropy have a layered perovskite crystal structure.
According to the method of producing a ceramic of the present invention, an ordinary baking furnace can be used for firing. When the same materials are used, an oriented ceramic having a higher orientation degree than that by the TGG method can be obtained. Therefore, the production cost of the ceramic can be reduced, and also, a ceramic having a higher orientation degree compared to that by the TGG method can be produced.
When the length of the oriented formed product in the direction parallel to the pressing axis is up to about half of the length of the formed product before pressing, a ceramic having a still higher orientation degree, e.g., an orientation degree substantially equal to that by the hot forging method, can be produced.
Moreover, when the amount of the ceramic crystal grains having a shape-anisotropy is in the range of about 25 to 52% by weight based on 100% by weight of the mixed powder, a ceramic having a high orientation degree and a high sintering density can be obtained.
Also, when the ceramic crystal grains having a shape-anisotropy are flat, and the aspect ratio (ratio of the maximum size thereof to the height) is in the range of about 5 to 10, a ceramic having a high orientation degree can be obtained. If the aspect ratio is higher than about 10, the density of the ceramic becomes low.
Moreover, when the ceramic crystal grains having a shape-anisotropy have a layered perovskite crystal structure, an oriented ceramic having a remarkably higher orientation degree and a superior piezoelectric characteristic can be produced. Examples of the material having the layered perovskite crystal structure includes BiWO6, CaBi2Nb2O9, SrBi2Nb2O9, BaBi2Nb2O9, PbBi2Nb2O9, CaBi2Ta2O9, SrBi2Ta2O9, BaBi2Ta2O9, PbBi2Ta2O9, Bi3TiNbO9, Bi3TiTaO9, Bi4Ti3O12, SrBi3Ti2NbO12, BaBi2Ti2NbO12, PbBi3Ti2NbO12, CaBi4Ti4O15, SrBi4Ti4O15, BaBi4Ti4O15, PbBi4Ti4O15, Na0.5Bi4.5Ti4O15, K0.5Bi4.5Ti4O16, Ca2Bi4Ti5O18, Sr2Bi4Ti5O18, Ba2Bi4Ti5O18, Pb2Bi4Ti5O18, BiTiWO18, Bi7Ti4NbO21, Bi10Ti2W3O30, and combinations of at least two of these materials.
The above-mentioned objects of the present invention, and also the characteristics and the advantages thereof will be clarified in the detailed description of the preferred modes of carrying out the invention and the examples of the invention, which is made hereinunder with reference to the drawings attached thereto.